Coatings based on alkyds are known to have good application and wetting properties, while coatings based on acrylic (latex) polymers generally have superior film properties such as durability, weathering, and chemical resistance. Accordingly, alkyd compositions modified with latex compositions would be useful to provide the specific advantages of each of the two coating types.
One way to accomplish this objective would be to combine in a physical blend an alkyd composition with a latex polymer composition. However, alkyd polymers and latex polymers are not readily compatible due to their distinctly different chemical structures and molecular weights. Accordingly, it has been determined that physical blends of alkyd polymers and latex polymers often separate, either in the blended composition itself or after the composition has been applied to a substrate. This separation is due, in part, to the difference in molecular weights of the two materials. Alkyds are generally prepared by polycondensation and typically have molecular weights below 10,000, while latex polymers are addition polymers and typically have molecular weights over 100,000. A blend may be prepared by using a latex polymer having a lower molecular weight. However, this generally results in poorer coating performance.
Modification of alkyd polymers has also been attempted by polymerizing latex monomers in the presence of alkyd dispersions, as disclosed in U.S. Pat. No. 4,451,596 and European Patent Application No. 0 555 903. Alkyd polymers prepared from these methods contain grafted moieties derived from the latex monomers. Coating compositions based on these materials generally have poor reactivity toward oxidative cure, due to insufficient amounts of unsaturated groups present in the fully formed latex grafted alkyd polymer. Many of the unsaturated moieties present in alkyd polymers are consumed during the free radical polymerization of the latex monomers, and are therefore not available for oxidative cure in the coatings compositions. Further, since the latex aspect of the alkyd polymer is derived from a monomer, as opposed to a latex polymer, the alkyd polymer cannot completely incorporate the beneficial aspects of the latex polymer due to the relatively low molecular weight of the grafted species.
In light of the above, it would be desirable to obtain a latex modified alkyd composition wherein the modified alkyd composition provides the benefits of both alkyd polymers and latex polymers in a stable composition.